Membrane probing system

ABSTRACT

A substrate, preferably constructed of a ductile material and a tool having the desired shape of the resulting device for contacting contact pads on a test device is brought into contact with the substrate. The tool is preferably constructed of a material that is harder than the substrate so that a depression can be readily made therein. A dielectric (insulative) layer, that is preferably patterned, is supported by the substrate. A conductive material is located within the depressions and then preferably lapped to remove excess from the top surface of the dielectric layer and to provide a flat overall surface. A trace is patterned on the dielectric layer and the conductive material. A polyimide layer is then preferably patterned over the entire surface. The substrate is then removed by any suitable process.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to probe assemblies of the typecommonly used for testing integrated circuits (IC) and, in particular,the present invention relates to a membrane probing assembly havingcontacts which scrub, in a locally controlled manner, across therespective input/output conductors of each device so as to reliably wipeclear the surface oxides that are normally found on those conductorsthereby ensuring good electrical connection between the probing assemblyand each device.

[0002] The trend in electronic production has been toward increasinglysmaller geometries particularly in integrated circuit technology whereina very large number of discrete circuit elements are fabricated on asingle substrate or “wafer.” After fabrication, this wafer is dividedinto a number of rectangular-shaped chips or “dice” where each diepresents a rectangular or other regular arrangement of metallizedcontact pads through which input/output connections are made. Althougheach die is eventually packaged separately, for efficiency sake, testingof the circuit formed on each die is preferably performed while the diesare still joined together on the wafer. One typical procedure is tosupport the wafer on a flat stage or “chuck” and to move the wafer in X,Y and Z directions relative to the head of the probing assembly so thatthe contacts on the probing assembly move from die to die forconsecutive engagement with each die. Respective signal, power andground lines are run to the probing assembly from the testinstrumentation thus enabling each circuit to be sequentially connectedto the test instrumentation.

[0003] One conventional type of probing assembly used for testingintegrated circuits provides contacts that are configured as needle-liketips. These tips are mounted about a central opening formed in a probecard so as to radially converge inwardly and downwardly through theopening. When the wafer is raised beyond that point where the pads onthe wafer first come into contact with these tips, the tips flexupwardly so as to skate for-wardly across their respective pads therebyremoving oxide buildup on the pads.

[0004] The problem with this type of probing assembly is that theneedle-like tips, due to their narrow geometry, exhibit high inductanceso that signal distortion is large in high frequency measurements madethrough these tips. Also, these tips can act in the manner of a planingtool as they wipe across their respective pads, thereby leading toexcessive pad damage. This problem is magnified to the extent that theprobe tips bend out of shape during use or otherwise fail to terminatein a common plane which causes the more forward ones of the tips to beardown too heavily on their respective pads. Also, it is impractical tomount these tips at less than 100 micron center-to-center spacing or ina multi-row grid-like pattern so as to accommodate the pad arrangementof more modern, higher density dies. Also, this type of probing assemblyhas a scrub length of the needle tips of 25 microns or more, whichincreases the difficulty of staying within the allowed probing area.

[0005] In order to reduce inductive losses, decrease pad wear andaccommodate smaller device geometries, a second type of probing assemblyhas been developed that uses a flexible membrane structure forsupporting the probing contacts. In this assembly, lead lines ofwell-defined geometry are formed on one or more plies of flexibleinsulative film, such as polyimide or MYLAR™. If separate plies areused, these plies are bonded together to form, for example, amultilayered transmission line structure. In the central portion of thisflexible structure or membrane, each conductive line is terminated by arespective probing contact which is formed on, and projects outwardlyfrom, an outer face of the membrane. These probing contacts are arrangedin a predetermined pattern that matches the pattern of the device padsand typically are formed as upraised bumps for probing the flat surfacesconventionally defined by the pads. The inner face of the membrane issupported on a supporting structure. This structure can take the form,for example, of a truncated pyramid, in which case the inner face of thecenter portion of the membrane is supported on the truncated end of thesupport while the marginal portions of the membrane are drawn away fromthe center portion at an angle thereto so as to clear any uprightcomponents that may surround the pads on the device.

[0006] With respect to the membrane probing assembly just described,excessive line inductance is eliminated by carefully selecting thegeometry of the lead lines, and a photolithographic process ispreferably used to enable some control over the size, spacing, andarrangement, of the probing contacts so as to accommodate higher densityconfigurations. However, although several different forms of thisprobing assembly have been proposed, difficulties have been encounteredin connection with this type of assembly in reducing pad wear and inachieving reliable clearing of the oxide layer from each of the devicepads so as to ensure adequate electrical connection between the assemblyand the device-under-test.

[0007] One conventional form of membrane probing assembly, for example,is exemplified shown in Rath European Patent Pub. No. 259,163A2. Thisdevice has the central portion of the sheet-like membrane mounteddirectly against a rigid support. This rigid support, in turn, isconnected by a resilient member comprising an elastomeric or rubberblock to the main body of the assembly so that the membrane can tilt tomatch the tilt of the device. Huff U.S. Pat. No. 4,918,383 shows aclosely related device wherein radially extending leaf springs permitvertical axis movement of the rigid support while preventing it fromtilting so that there is no slippage or “misalignment” of the contactbumps on the pads and further so that the entire membrane will shiftslightly in the horizontal plane to allow the contacts to “scrub” acrosstheir respective pads in order to clear surface oxides from these pads.

[0008] In respect to both of these devices, however, because ofmanufacturing tolerances, certain of the contact bumps are likely to bein a recessed position relative to their neighbors' and these recessedbumps will not have a satisfactory opportunity to engage their padssince they will be drawn away from their pads by the action of theirneighbors on the rigid support. Further-more, even when “scrub” movementis provided in the manner of Huff, the contacts will tend tofrictionally cling to the device as they perform the scrubbing movement,that is, there will be a tendency for the pads of the device to move inunison with the contacts so as to negate the effect of the contactmovement. Whether any scrubbing action actually occurs depends on howfar the pads can move, which depends, in turn, on the degree of lateralplay that exists as a result of normal tolerance between the respectivebearing surfaces of-the probe head and chuck. Hence this form ofmembrane probing assembly does not ensure reliable electrical connectionbetween each contact and pad.

[0009] A second conventional form of membrane probing assembly isexemplified by the device shown in Barsotti European Patent Pub. No.304,868A2. This device provides a flexible backing for the central orcontact-carrying portion of the flexible membrane. In Barsotti, themembrane is directly backed by an elastomeric member and this member, inturn, is backed by a rigid support so that minor height variationsbetween the contacts or pads can be accommodated. It is also possible touse positive-pressure air, negative-pressure air, liquid or an unbackedelastomer to provide flexible backing for the membrane, as shown inGangroth U.S. Pat. No. 4,649,339, Ardezzone U.S. Pat. No. 4,636,772,Reed, Jr. et al. U.S. Pat. No. 3,596,228 and Okubo et al. U.S. Pat. No.5,134,365, respectively. These alternative devices, however, do notafford sufficient pressure between the probing contacts and the devicepads to reliably penetrate the oxides that form on the pad surfaces.

[0010] In this second form of membrane probing assembly, as indicated inOkubo, the contacts may be limited to movement along the Z-axis in orderto prevent slippage and resulting misalignment between the contacts andpads during engagement. Thus, in Barsotti, the rigid support underlyingthe elastomeric member is fixed in position although it is also possibleto mount the support for Z-axis movement in the manner shown in HuffU.S. Pat. No. 4,980,637. Pad damage is likely to occur with this type ofdesign, however, because a certain amount of tilt is typically presentbetween the contacts and the device, and those contacts angled closestto the device will ordinarily develop much higher contact pressures thanthose which are angled away. The same problem arises with the relatedassembly shown in European Patent Pub. No. 230,348A2 to Garretson, eventhough in the Garretson device the characteristic of the elastomericmember is such as to urge the contacts into lateral movement when thosecontacts are placed into pressing engagement with their pads. Yetanother related assembly is shown in Evans U.S. Pat. No. 4,975,638 whichuses a pivotably mounted support for backing the elastomeric member soas to accommodate tilt between the contacts and the device. However, theEvans device is subject to the friction clinging problem alreadydescribed insofar as the pads of the device are likely to cling to thecontacts as the support pivots and causes the contacts to shiftlaterally.

[0011] Yet other forms of conventional membrane probing assemblies areshown in Crumly U.S. Pat. No. 5,395,253, Barsotti et al. U.S. Pat. No.5,059,898 and Evans et al. U.S. Pat. No. 4,975,638. In Crumly, thecenter portion of a stretchable membrane is resiliently biased to afully stretched condition using a spring. When the contacts engage theirrespective pads, the stretched center portion retracts against thespring to a partially relaxed condition so as to draw the contacts inradial scrub directions toward the center of the membrane. In Barsotti,each row of contacts is supported by the end of a respective L-shapedarm so that when the contacts in a row engage their respective pads, thecorresponding arm flexes upwardly and causes the row of contacts tolaterally scrub simultaneously across their respective pads. In bothCrumly and Barsotti, however, if any tilt is present between thecontacts and the device at the time of engagement, this tilt will causethe contacts angled closest to the device to scrub further than thoseangled further away. Moreover, the shorter contacts will be forced tomove in their scrub directions before they have had the opportunity toengage their respective pads due to the controlling scrub action oftheir neighboring contacts. A further disadvantage of the Crumly device,in particular, is that the contacts nearer to the center of the membranewill scrub less than those nearer to the periphery so that scrubeffectiveness will vary with contact position.

[0012] In Evans et al. U.S. Pat. No. 5,355,079 each contact constitutesa spring metal finger, and each finger is mounted so as to extend in acantilevered manner away from the underlying membrane at a predeterminedangle relative to the membrane. A similar configuration is shown inHiggins U.S. Pat. No. 5,521,518. It is difficult, however, to originallyposition these fingers so that they all terminate in a common plane,particularly if a high density pattern is required. Moreover, thesefingers are easily bent out of position during use and cannot easily berebent back to their original position. Hence, certain ones of thefingers are likely to touch down before other ones of the fingers, andscrub pressures and distances are likely to be different for differentfingers. Nor, in Evans at least, is there an adequate mechanism fortolerating a minor degree of tilt between the fingers and pads. AlthoughEvans suggests roughening the surface of each finger to improve thequality of electrical connection, this roughening can cause undueabrasion and damage to the pad surfaces. Yet a further disadvantage ofthe contact fingers shown in both Evans and Higgins is that such fingersare subject to fatigue and failure after a relatively low number of“touchdowns” or duty cycles due to repeated bending and stressing.

[0013] Referring to FIG. 1, Cascade Microtech, Inc. of Beaverton, Oregonhas developed a probe head 40 for mounting a membrane probing assembly42. In order to measure the electrical performance of a particular diearea 44 included on the silicon wafer 46, the high-speed digital lines48 and/or shielded transmission lines 50 of the probe head are connectedto the input/output ports of the test instrumentation by a suitablecable assembly, and the chuck 51 which supports the wafer is moved inmutually perpendicular X,Y,Z directions in order to bring the pads ofthe die area into pressing engagement with the contacts included on thelower contacting portion of the membrane probing assembly.

[0014] The probe head 40 includes a probe card 52 on which thedata/signal lines 48 and 50 are arranged. Referring to FIGS. 2-3, themembrane probing assembly 42 includes a support element 54 formed ofincompressible material such as a hard polymer. This element isdetachably connected to the upper side of the probe card by four Allenscrews 56 and corresponding nuts 58 (each screw passes through arespective attachment arm 60 of the support element, and a separatebacking element 62 evenly distributes the clamping pressure of thescrews over the entire back side of the supporting element). Inaccordance with this detachable connection, different probing assemblieshaving different contact arrangements can be quickly substituted foreach other as needed for probing different devices.

[0015] Referring to FIGS. 3-4, the support element 54 includes arearward base portion 64 to which the attachment arms 60 are integrallyjoined. Also included on the support element 54 is a forward support orplunger 66 that projects outwardly from the flat base portion. Thisforward support has angled sides 68 that converge toward a flat supportsurface 70 so as to give the forward support the shape of a truncatedpyramid. Referring also to FIG. 2, a flexible membrane assembly 72 isattached to the support after being aligned by means of alignment pins74 included on the base portion. This flexible membrane assembly isformed by one or more plies of insulative sheeting such as KAPTON™ soldby E.I. Du Pont de Nemours or other polyimide film, and flexibleconductive layers or strips are provided between or on these plies toform the data/signal lines 76.

[0016] When the support element 54 is mounted on the upper side of theprobe card 52 as shown in FIG. 3, the forward support 66 protrudesthrough a central opening 78 in the probe card so as to present thecontacts which are arranged on a central region 80 of the flexiblemembrane assembly in suitable position for pressing engagement with thepads of the device under test. Referring to FIG. 2, the membraneassembly includes radially extending arm segments 82 that are separatedby inwardly curving edges 84 that give the assembly the shape of aformee cross, and these segments extend in an inclined manner along theangled sides 68 thereby clearing any upright components surrounding thepads. A series of contact pads 86 terminate the data/signal lines 76 sothat when the support element is mounted, these pads electrically engagecorresponding termination pads provided on the upper side of the probecard so that the data/signal lines 48 on the probe card are electricallyconnected to the contacts on the central region.

[0017] A feature of the probing assembly 42 is its capability forprobing a somewhat dense arrangement of contact pads over a large numberof contact cycles in a manner that provides generally reliableelectrical connection between the contacts and pads in each cycledespite oxide buildup on the pads. This capability is a function of theconstruction of the support element 54, the flexible membrane assembly72 and their manner of interconnection. In particular, the membraneassembly is so constructed and connected to the support element that thecontacts on the membrane assembly preferably wipe or scrub, in a locallycontrolled manner, laterally across the pads when brought into pressingengagement with these pads. The preferred mechanism for producing thisscrubbing action is described in connection with the construction andinterconnection of a preferred membrane assembly 72 a as best depictedin FIGS. 6 and 7a-7 b.

[0018]FIG. 6 shows an enlarged view of the central region 80 a of themembrane assembly 72 a. In this embodiment, the contacts 88 are arrangedin a-square-like pattern suitable for engagement with a square-likearrangement of pads. Referring also to FIG. 7a, which represents asectional view taken along lines 7 a-7 a in FIG. 6, each contactcomprises a relatively thick rigid beam 90 at one end of which is formeda rigid contact bump 92. The contact bump includes thereon a contactingportion 93 which comprises a nub of rhodium fused to the contact bump.Using electroplating, each beam is formed in an overlapping connectionwith the end of a flexible conductive trace 76 a to form a jointtherewith. This conductive trace in conjunction with a back-planeconductive layer 94 effectively provides a controlled impedancedata/signal line to the contact because its dimensions are establishedusing a photolithographic process. The backplane layer preferablyincludes openings therein to assist, for example, with gas ventingduring fabrication.

[0019] The membrane assembly is interconnected to the flat supportsurface 70 by an interposed elastomeric layer 98, which layer iscoextensive with the support surface and can be formed by a siliconerubber compound such as ELMER'S STICK-ALL™ made by the Borden Company orSylgard 182 by Dow Corning Corporation. This compound can beconveniently applied in a paste-like phase which hardens as it sets. Theflat support surface, as previously mentioned, is made of incompressiblematerial and is preferably a hard dielectric such as polysulfone orglass.

[0020] In accordance with the above-described construction, when one ofthe contacts 88 is brought into pressing engagement with a respectivepad 100, as indicated in FIG. 7b, the resulting off-center force on therigid beam 90 and bump 92 structure causes the beam to pivot or tiltagainst the elastic recovery force provided by the elastomeric pad 98.This tilting motion is localized in the sense that a forward portion 102of the beam moves a greater distance toward the flat support surface 70than a rearward portion 104 of the same beam. The effect is such as todrive the contact into lateral scrubbing movement across the pad as isindicated in FIG. 7b with a dashed-line and solid-line representationshowing the beginning and ending positions, respectively, of the contacton the pad. In this fashion, the insulative oxide buildup on each pad isremoved so as to ensure adequate contact-to-pad electrical connections.

[0021]FIG. 8 shows, in dashed line view, the relative positions of thecontact 88 and pad 100 at the moment of initial engagement or touchdownand, in solid-line view, these same elements after “overtravel” of thepad by a distance 106 in a vertical direction directly toward the flatsupport surface 70. As indicated, the distance 108 of lateral scrubbingmovement is directly dependent on the vertical deflection of the contact88 or, equivalently, on the overtravel distance 106 moved by the pad100. Hence, since the overtravel distance for each contact on thecentral region 80 a will be substantially the same (with differencesarising from variations in contact height), the-distance of lateralscrubbing movement by each contact on the central region will besubstantially uniform and will not, in particular, be affected by therelative position of each contact on the central region.

[0022] Because the elastomeric layer 98 is backed by the incompressiblesupport surface 70, the elastomeric layer exerts a recovery force oneach tilting beam 90 and thus each contact 93 to maintain contact-to-padpressure during scrubbing. At the same time, the elastomeric layeraccommodates some height variations between the respective contacts.Thus, referring to FIG. 9a, when a relatively shorter contact 88 a issituated between an immediately adjacent pair of relatively tallercontacts 88 b and these taller contacts are brought into engagement withtheir respective pads, then, as indicated in FIG. 9b, deformation by theelastomeric layer allows the smaller contact to be brought intoengagement with its pad after some further overtravel by the pads. Itwill be noted, in this example, that the tilting action of each contactis locally controlled, and the larger contacts are able, in particular,to tilt independently of the smaller contact so, that the smallercontact is not urged into lateral movement until it has actually toucheddown on its pad.

[0023] Referring to FIGS. 10 and 11, the electroplating process toconstruct such a beam structure, as schematically shown in FIG. 8,includes the incompressible material 68 defining the support surface 70and the substrate material attached thereon, such as the elastomericlayer 98. Using a flex circuit construction technique, the flexibleconductive trace 76 a is then patterned on a sacrificial substrate.Next, a polyimide layer 77 is patterned to cover the entire surface ofthe sacrificial substrate and of the traces 76 a, except for the desiredlocation of the beams 90 on a portion of the traces 76 a. The beams 90are then electroplated within the openings in the polyimide layer 77.Thereafter, a layer of photoresist 79 is patterned on both the surfaceof the polyimide 77 and beams 90 to leave openings for the desiredlocation of the contact bumps 92. The contact bumps 92 are thenelectroplated within the openings in the photoresist layer 79. Thephotoresist layer 79 is removed and a thicker photoresist layer 81 ispatterned to cover the exposed surfaces, except for the desiredlocations for the contacting portions 93. The contacting portions 93 arethen electroplated within the openings in the photoresist layer 81. Thephotoresist layer 81 is then removed. The sacrificial substrate layer isremoved and the remaining layers are attached to the elastomeric layer98. The resulting beams 90, contact bumps 92, and contacting portions93, as more accurately illustrated in FIG. 12, provides the independenttilting and scrubbing functions of the device.

[0024] Unfortunately, the aforementioned construction technique resultsin a structure with many undesirable characteristics.

[0025] First, several beams 90, contact bumps 92, and contactingportions 93 (each of which may be referred to as a device) proximate oneanother results in different localized current densities within theelectroplating bath, which in turn results in differences in the heightsof many of the beams 90, contact bumps 92, and contacting portions 93.Also, different densities of the ions with-in the electroplating bathand “random” variations in the electroplating bath also results indifferences in heights of many of the beams 90, contact bumps 92, andcontacting portions 93. The different heights of many of the beams 90,contact bumps 92, and contacting portions 93 is compounded three fold inthe overall height of many of the devices. Accordingly, many deviceswill have a significantly different height than other devices. Usingmembrane probes having variable device height requires more pressure toensure that all the contacting portions 93 make adequate contact withthe test device than would be required if all the devices had equaloverall height. For high density membrane probes, such as 2000 or moredevices in a small area, the cumulate effect of the additional pressurerequired for each device may exceed the total force permitted for theprobe head and probe station. The excess pressure may also result inbending and breaking of the probe station, the probe head, and/or themembrane probing assembly. In addition, the devices with the greatestheight may damage the pads on the test device because of the increasedpressure required to make suitable contact for the devices with thelowest height.

[0026] Second, the ability to decrease the pitch (spacing) between thedevices is limited by the “mushrooming” effect of the electroplatingprocess over the edges of the polyimide 77 and photoresist layers 79 and81. The “mushrooming” effect is difficult to control and results in avariable width of the beams 90, contact bumps 92, and contactingportions 93. If the height of the beams 90, the contact bumps 92, or thecontacting portions 93 are increased then the “mushrooming” effectgenerally increases, thus increasing the width of the respectiveportion. The increased width of one part generally results in a wideroverall device which in turn increases the minimum spacing betweencontacting portions 93. Alternatively, decreasing the height of thebeams 90, the contact bumps 92, or the contacting portions 93 generallydecreases the width of the “mushrooming” effect which in turn decreasesthe minimum spacing between contacting portions 93. However, if theheight of the contacting portions 93 relative to the respective beam 90is sufficiently reduced, then during use the rearward end of the beam 90may sufficiently tilt and contact the test device in an acceptablelocation, i.e., off the contact pad.

[0027] Third, it is difficult to plate a second metal layer directly ontop of a first metal layer, such as contacting portions 93 on thecontact bumps 92, especially when using nickel. To provide a bondbetween the contact bumps 92 and the contacting portions 93, aninterface seed layer such as copper or gold is used to make an improvedinterconnection. Unfortunately, the interface seed layer reduces thelateral strength of the device due to the lower sheer strength of theinterface layer.

[0028] Fourth, applying a photoresist layer over a non-uniform surfacetends to be semi-conformal in nature resulting in a non-uniformthicknesses of the photoresist material itself. Referring to FIG. 13,the photoresist layer 79 (and 81) over the raised portions of the beams90 tends to be thicker than the photoresist layer 79 (and 81) over thelower portions of the polyimide 77. In addition, the thickness of thephotoresist 79 (and 81) tends to vary depending on the density of thebeams 90. Accordingly, regions of the membrane probe that have a denserspacing of devices, the photoresist layer 79 (and 81) will be thicker onaverage than regions of the membrane probe that have a less densespacing of devices. During the exposing and etching processing of thephotoresist layer 79 (and 81), the duration of the process depends onthe thickness of the photoresist 79 (or 81). With variable photoresistthickness it is difficult to properly process the photoresist to provideuniform openings. Moreover, the thinner regions of photoresist layer 79(or 81) will tend to be overexposed resulting in variably sizedopenings. Also, the greater the photoresist layer thickness 79 (or 81)the greater the variability in its thickness. Accordingly, the use ofphotoresist presents many processing problems.

[0029] Fifth, separate alignment processes are necessary to align thebeams 90 on the traces 76 a, the contact bumps 92 on the beams 90, andthe contacting portions 93 on the contact bumps 92. Each alignmentprocess has inherent variations that must be accounted for in sizingeach part. The minimum size of the contacting portions 93 is definedprimarily by the lateral strength requirements and the maximum-allowablecurrent density therein. The minimum size of the contacting portions 93,accounting for the tolerances in alignment, in turn defines the minimumsize of the contact bumps 92 so that the contacting portions 93 aredefinitely constructed on the contact bumps 92. The minimum size of thecontact bumps 92, in view of the contacting portions 93 and accountingfor the tolerances in alignment, defines the minimum size of the beams90 so that the contact bumps 92 are definitely constructed on the beams90. Accordingly, the summation of the tolerances of the contact bumps 92and the contacting portions 93, together with a minimum size of thecontacting portions 93, defines the minimum device size, and thusdefines the minimum pitch between contact pads.

[0030] What is desired, therefore, is a membrane probe constructiontechnique and structure that results in a more uniform device height,decreased spacing between devices, maximized lateral strength, desiredgeometries, and proper alignment.

SUMMARY OF THE INVENTION

[0031] The present invention overcomes the aforementioned drawbacks ofthe prior art by providing a substrate, preferably constructed of aductile material. A tool having the desired shape of the resultingdevice for contacting contact pads on a test device is brought intocontact with the substrate. The tool is preferably constructed of amaterial that is harder than the substrate so that a depression can bereadily made therein. A dielectric (insulative) layer, that ispreferably patterned, is supported by the substrate. A conductivematerial is located within the depressions and then preferablyplanarized to remove excess from the top surface of the dielectric layerand to provide a flat overall surface. A trace is patterned on thedielectric layer and the conductive material. A polyimide layer is thenpreferably patterned over the entire surface. The substrate is thenremoved by any suitable process.

[0032] The foregoing and other objectives, features, and advantages ofthe invention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0033]FIG. 1 is a perspective view of a membrane probing assembly boltedto a probe head and a wafer supported on a chuck in suitable positionfor probing by this assembly.

[0034]FIG. 2 is a bottom elevational view showing various parts of theprobing assembly of FIG. 1, including a support element and flexiblemembrane assembly, and a fragmentary view of a probe card havingdata/signal lines connected with corresponding lines on the membraneassembly.

[0035]FIG. 3 is a side elevational view of the membrane probing assemblyof FIG. 1 where a portion of the membrane assembly has been cut away toexpose hidden portions of the support element.

[0036]FIG. 4 is a top elevational view of an exemplary support element.

[0037]FIGS. 5a-5 b are schematic side elevational views illustrating howthe support element and membrane assembly are capable of tilting tomatch the orientation of the device under test.

[0038]FIG. 6 is an enlarged top elevational view of the central regionof the construction of the membrane assembly of FIG. 2.

[0039]FIGS. 7a-7 b are sectional views taken along lines 7 a-7 a in FIG.6 first showing a contact before touchdown and then showing the samecontact after touchdown and scrub movement across its respective pad.

[0040]FIG. 8 is a schematic side view showing, in dashed-linerepresentation, the contact of FIGS. 7a-7 b at the moment of initialtouchdown and, in solid-line representation, the same contact afterfurther vertical overtravel by the pad.

[0041]FIGS. 9a and 9 b illustrate the deformation of the elastomericlayer to bring the contacts into contact with its pad.

[0042]FIG. 10 is a longitudinal sectional view of the device of FIG. 8.

[0043]FIG. 11 is a cross sectional view of the device of FIG. 8.

[0044]FIG. 12 is a more accurate pictorial view of the device shown inFIGS. 10 and 11.

[0045]FIG. 13 is a detailed view of the device shown in FIG. 11illustrating the uneven layers that result during processing.

[0046]FIG. 14 is a pictorial view of a substrate.

[0047]FIG. 15 is a pictorial view of an exemplary embodiment of a tool,and in particular a dimpling tool, of the present invention.

[0048]FIG. 16 is a pictorial view illustrating the tool of FIG. 15coming into contact with the substrate of FIG. 14.

[0049]FIG. 17 is a pictorial view of the substrate of FIG. 14 after thetool of FIG. 15 has come into contact therewith.

[0050]FIG. 18 is a sectional view of the substrate of FIG. 14 with apolyimide layer supported thereon.

[0051]FIG. 19 is a pictorial view of the tool of FIG. 16 together with az-axis stop.

[0052]FIG. 20 is a sectional view of the substrate of FIG. 14 with atrace, conductive material in the depression, and additional polyimidelayer thereon.

[0053]FIG. 21 is a pictorial view of the device of FIG. 20, inverted,with the substrate removed.

[0054]FIG. 22 is a breakaway sectional view of the contacting portion ofFIG. 21.

[0055]FIG. 23 is a schematic view illustrating one arrangement of thedevices of the present invention.

[0056]FIG. 24 is a schematic view illustrating the contact of atraditional contacting portion and the oxide layer of a solder bump.

[0057]FIG. 25 is a plan view of an alternative device with an elongateprobing portion.

[0058]FIG. 26 is a side view of the device of FIG. 25 with an elongateprobing portion.

[0059]FIG. 27 is a pictorial view of a solder bump with a mark thereinas a result of the device of FIGS. 25 and 26.

[0060]FIG. 28 is a pictorial view of another alternative probing device.

[0061]FIG. 29 is a pictorial view of a further alternative probingdevice suitable for solder bumps.

[0062]FIG. 30 is a side view of a true Kelvin connection using thedevices of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0063] The currently employed construction techniques for membraneprobes involves starting with the flat rigid substrate to supportadditional layers fabricated thereon. To decrease the pitch and providedevices with increased uniformity requires increasingly more complex andexpensive processing techniques. In direct contrast to the currenttechniques of constructing layers from the “bottom up” upon a supportingsubstrate, the present inventors came to the realization that by using asuitable tool a substrate may be coined to create the desired beams,contact bumps, and contacting portions. The remaining layers are thenconstructed “top down” on the beam. The substrate itself is thereafterremoved.

[0064] Referring to FIG. 14, a substrate 200 is preferably constructedfrom a ductile material such as aluminum, copper, lead, indium, brass,gold, silver, platinum, or tantalum, with a thickness preferably between10 mills and ⅛inch. The top surface 202 of the substrate 200 ispreferably planar and polished for optical clarity to improve-viewing,as described later.

[0065] Referring to FIG. 15, a tool and in particular a “dimpling” tool210 is constructed with a head 212 having the desired shape of theresulting device for contacting the contact pads on the test device. Thedimpling tool 210 includes a projection 214 to connect to a dimplingmachine (not shown). The tool 210 is supported by the dimpling machinewith the head 212 oriented to come into contact with the top surface 202of the substrate 200. The tool 210 is preferably constructed of amaterial that is harder than the substrate 200 so that a dimple can bereadily made therein. Suitable material for the tool 210 is, forexample, tool steel, carbide, chromium, and diamond. The preferreddimpling machine is a probe station which has accurate x, y, and zcontrol.

[0066] It is to be understood that any other suitable dimpling machinemay likewise be used. Referring to. FIG. 16, the tool 210 is pressedinto contact with the top surface 202 of the substrate 200 resulting ina depression 216 matching the shape of the tool 210 upon its removalfrom the substrate 200, as shown in FIG. 17. The tool 210 is used tocreate a plurality of depressions 216 in the substrate 200 matching thedesired pattern, such as the pattern shown in FIG. 6. Conversely, thetool 210 can be held stationary and the substrate 200 can be moved inthe z-direction until the top surface 202 of the substrate is pressedinto contact with the tool 210 resulting in the same depression 216matching the shape of the tool 210 upon its removal from the substrate200, as shown in FIG. 17.

[0067] Referring to FIG. 18, a polyimide layer 220 is patterned aroundthe depressions 216. It is to be understood that any other suitableinsulative layer or dielectric layer may likewise be used. In theprocess of patterning the polyimide layer 220, it is somewhat difficultto remove the polyimide from the depressions 216 during the exposing andetching process for the polyimide layer 220. This is especially truewhen the depressions 216 are relatively deep with steeply inclinedsides. Alternatively, the polyimide layer 220 may be patterned on thetop surface 202 of the substrate 200 with openings located therein wherethe depressions 216 are desired. Thereafter, the tool 210 is used tocreate the depressions 216 in the substrate 200 through the openingsprovided in the polyimide layer 220. This alternative techniqueeliminates the difficult process of adequately removing the polyimidelayer 220 from the depressions 216.

[0068] It is expensive to manufacture masks for exposing the polyimidelayer 220 that have tolerances sufficient to precisely align theopenings for the depressions 216. The tool 210, in combination with thedimpling machine, can be aligned to the actual location of one of theopenings that results from exposing and etching the polyimide layer 220with a relatively inexpensive, and somewhat inaccurate mask. The presentinventors came to the realization that localized regions of the mask,and thus the openings resulting therefrom, tend to be relatively wellaligned for purposes of dimpling. Likewise, regions of the mask distantfrom one another tend not to be relatively well aligned for purposes ofdimpling. Accordingly, automatically dimpling the substrate 200 to matchan anticipated pattern with many depressions 216 distant from oneanother, with an accurate dimpling machine, will result in the dimplingtool not accurately being aligned with the openings at regions distantfrom the initial alignment point. To improve the accuracy of thealignment process the present inventors came to the realization that thedimpling machine may be realigned to the actual openings in thepolyimide layer 220 at different remote locations, so that eachlocalized region is relatively accurately aligned, while the overallalignment may be somewhat off. In this manner a relatively inexpensivemask may be used.

[0069] Preferably the dimpling machine includes accurate z-axis movementso that the depth of each depression is identical, or substantiallyidentical. Referring to FIG. 19, if sufficiently accurate z-axismovement is not available then an alternative dimpling tool 240 with abuilt in z-axis stop 242 may be used. The z-axis stop 242 is aprojection extending outward from the head 244 that comes to rest on thetop surface of the polyimide 220 or top surface 202 of the substrate200. The z-axis stop 242 is positioned with respect to the head 244 suchthat the proper depth is obtained, taking into account whether or notthe polyimide layer 220 is previously patterned before using thedimpling tool 240.

[0070] Referring to FIG. 20, a conductive material 250 is electroplatedonto the polyimide 220 and substrate 200 thereby filling up thedepressions 216 with the conductive material 250, such as nickel andrhodium. It is to be understood that any other suitable technique may beused to locate conductive material within the depressions 216. Theconductive material 250 is then preferably lapped to remove excess fromthe top surface of the polyimide layer 220 and to provide a flat overallsurface. The preferred lapping process is a chemical-mechanicalplanarization process. A trace 252 is patterned on the polyimide layer220 and the conductive material 250. The trace 252 is preferably a goodconductor such as copper, aluminum, or gold. A polyimide layer 254 isthen patterned over the entire surface. Further layers of metal anddielectric may be formed. The substrate 200 is then removed by anysuitable process, such as etching with hydrochloric acid (HCL 15%) orsulfuric acid (H₂SO₄). Hydrochloric acid and sulfuric acid are notreactive with the polyimide layer 220 nor the conductive material 250,such as nickel or rhodium. It is to be understood that the polyimidelayer 254 may alternatively be any suitable insulator or dielectriclayer.

[0071] Referring to FIG. 21, the contacting portion 260 of the resultingdevice is preferably selected to have a low contact resistance so that agood electrical connection may be made with the test device. Whilenickel has a relatively low contact resistance, rhodium has an evenlower contact resistance and is more resistant to wear than nickel.Accordingly, the depressions 216 are preferably coated with a layer ofrhodium. Using normal processing techniques the thickness of rhodium islimited to approximately 5 microns. The resulting device includes anexterior layer of rhodium, and in particular the contacting portion 260,which is then filled with the remaining conductive material, such asnickel or a nonconductive fill. The conductive material need not fillthe entire depression.

[0072] The aforementioned “top-down” construction process providesnumerous advantages over the traditional “bottom-up” processingtechnique of constructing layers upon a supporting substrate. Theseadvantages also permit the capability of constructing devices withimproved characteristics.

[0073] First, there are no limitations to the height of the resultingdevices which were previously imposed by limitations of photoresistprocessing. The ability to construct devices having any suitable heightalso relieves the limitations imposed by attempting to electroplate intoa tall narrow openings in photoresist, which is difficult.

[0074] Second, the elevation of the contacting portions 260 of thedevices is extremely uniform because it is defined solely by the toolingprocess, which is mechanical in nature. Different localized currentdensities of the electroplating bath, different densities of the ionswithin the electroplating bath, and “random” variations in theelectroplating bath are eliminated from impacting the overall shape andheight of the resulting devices. With substantially uniform elevation ofthe devices, less force is required for the devices to make adequatecontact with the test device which, in turn, decreases the likelihood ofbending and breaking the probe station, the probe head, and/or themembrane probing assembly. Also, the substantially uniform elevation ofthe devices decreases the likelihood of damaging contact pads on thetest device with excessive pressure.

[0075] Third, the contacting portion 260 of the devices are strongerbecause the device is constructed of a single homogenous material duringone depositing process requiring no interfacial layers, as previouslyrequired for the multiple processing steps. This permits reducing thesize of the contacting portions to the limitation of the maximum currentdensity allowable therein during testing and not the minimum sheer forceof the interfacial layers.

[0076] Fourth, the shape of the resulting devices are customizable toeffectively probe different materials. The shape of the device may havesteep sidewall angles, such as 85 degrees, while still providingmechanical strength, stability, and integrity. The steep sidewallspermit narrower devices to be constructed which allows or a greaterdensity of devices for increasingly denser arrangements of contact padson the test device. Moreover, the angle of the sidewalls are notdependent (e.g. independent) on the crystalline structure of thesubstrate.

[0077] Fifth, the shape of the contacting portion is known precisely,and is uniform between devices, which permits uniform contact with thecontact pads of the test device.

[0078] Sixth, the alignment of the different portions of the resultingdevice are exactly uniform between devices because each device wasconstructed using the same tooling process. With exact alignment of thelower portions of each device (beam and contact bump) in relation to thecontacting portion, there is no need to provide additional leeway toaccommodate processing variations inherent in photoresist processes andin electroplating processes. Also, the “mushrooming” effect of theelectroplating process is eliminated which also reduces the requiredsize of the device. The alignment variability reduction, and virtualelimination, of different devices 300 allows a significantly decreasedpitch to be obtained, suitable for contact pads on the test device thathave increased density.

[0079] Seventh, the shape of the resulting devices may be tailor shapedto provide optimal mechanical performance. To provide the scrubbingfunction, as described in the background portion, the device should havea beam and bump structure that tilts upon contact. The device 300 mayinclude an inclined surface 304 between its tail 302 and the contactingportion 260. The inclined surface 304 provides for increased strengthalong portions of the length of the device 300 which permits the tail302 to be thinner than its head 306. The torque forces applied to thedevice 300 during the tilting process of the device 300 tend to decreaseover the length of the device 300 which has a correspondingly thinnermaterial defined by the inclined surface 304. With a thinner tail 302and material proximate the tail 302, the tail 302 of the device 300 hasless likelihood of impacting the test device if excess tiling occurs.The improved shape of the device 300 also decreases the amount of metalmaterial required.

[0080] Eighth, “look-up” cameras are used to obtain an image of thelower portion of the membrane probe to determine the precise location ofthe devices 300 relative to the contact pads on the test device. Using“look-up” cameras permits automatic alignment of the membrane devicesrelative to the contact pads so that automatic testing may be performed.In order to obtain an image of the devices 300 on the membrane probe the“look-up” cameras normally utilize light to illuminate the devices 300.Unfortunately, the traditional planar processing techniques result inrelatively flat surfaces on the beams, contact bumps, and contactingportions, in a perpendicular orientation to the “look up” cameras eachof which reflects light back to the “look-up” camera. The lightreflecting back to the “look up” camera from all the surfaces frequentlyresults in some confusion regarding the exact location of the contactingportions 260. The inclined surface 304 of the devices 300 tends toreflect incident light away from lowerly disposed “look-up” cameras,while the contacting portions 306 tend to reflect incident light back tolowerly disposed “look-up” cameras. Light returning to the “look-up”camera primarily from the contacting portions 306 results in lesspotential confusion regarding the exact location of the contactingportions.

[0081] Ninth, the initial polishing of the top surface 202 of thesubstrate 200 results in a matching smooth lower surface for thepolyimide layer 220 patterned thereon. After etching away, or otherwiseremoving, the substrate 200 the lower surface of the polyimide layer 220is smooth and the resulting polyimide layer 220 is generally opticallyclear. Accordingly, the spaces between the traces and the metallizeddevices 300 is relatively optically transmissive so that an operatorpositioning the device can readily see through the device between thetraces and devices. This assists the operator in manually positioningthe membrane probe on the devices which are otherwise obscured. Inaddition, the pyramidal shape of the devices 300 allows the operator tomore easily determine the exact location of the contacting portionsrelative to the contact pads on the test device, which were previouslyobscured by the wide beam structures (relative to the contactingportions).

[0082] Tenth, referring to FIG. 22, the contacting portions 260 of thedevice are preferably constructed with an exterior surface of rhodium340, which typically can be effectively plated to only approximately athickness of 5 microns. The plating process of rhodium issemi-conformal, so the resulting layer is approximately 5 microns thickin a perpendicular direction to the exterior sides 352 and 354. Thewidth of the top 350 of the contacting portion and the angle of thesides 352 and 354 of the tool 210 is selected so that the rhodium 340plated on both sides 352 and 254 preferably join together forming av-shape. The remainder of the device is preferably nickel. While thethickness of the rhodium 340 is only 5 microns in a perpendiculardirection, the thickness of the rhodium 340 in a perpendicular directionfrom the top 350 of the device is greater than 5 microns. Accordingly,the contacting portion which wears during use in a generallyperpendicular direction from the top 350 will last longer than if thetop portion were merely plated to a thickness of 5 microns of rhodium.

[0083] Eleventh, the texture of the contacting portion 260 may beselected to provide the described scrubbing effect on the contact padsof the test device. In particular, the tool may include a roughenedsurface pattern on the corresponding contacting portion to provide auniform texture for all devices.

[0084] Thirteenth, using the construction technique of the presentinvention is relatively quick to construct the devices because of thedecreased number of processing steps, resulting in a substantial costsavings.

[0085] The aforementioned construction technique also provides severaladvantages related to the shape of the devices which would be otherwisedifficult, if not impossible, to construct.

[0086] First, the tool may provide any desired shape, such as a simplebump, if no scrubbing action is desired.

[0087] Second, the inclined supporting sides of the test device up tothe contacting portion 260 provides superior mechanical support for thecontacting portion 260, as opposed to merely a portion of metalsupported by a larger contact bump. With such support from the inclinedsides, the contacting portion may be smaller without risk of it becomingdetached from the device. The smaller contacting portion providesimproved contact with the contact pad of the test device when the devicetilts to penetrate the oxide buildup on the surface of the contact pad.In addition, the tail 302 of the device may be substantially thinnerthan the remainder of the device which decreases the likelihood of thetail 302 portion impacting the contact pad of the test device duringtesting when the device tilts.

[0088] Third, the pressure exerted by the contacting portions of thedevices, given a predefined pressure exerted by the probe head, isvariable by changing the center of rotation of the device. The center ofrotation of the device can be selected by selecting the length of thedevice and the location/height of the contacting portion relativethereto. Accordingly, the pressures can 35 be selected, as desired, tomatch characteristics of two different contact pads.

[0089] Fourth, referring to FIG. 23, a triangular shape of the footprintof the device allows for high lateral stability of the devices whilepermitting a decrease in the pitch between devices. The contactingportions 403 of the device are preferably aligned in a lineararrangement for many contact pads of test devices. The triangularportions of the device are aligned in alternatively opposing directions.

[0090] Fifth, the capability of constructing contacting portions thatare raised high from the lower surface of the device, while stillmaintaining uniformity in the device height and structural strength,allows the device to provide scrubbing action while the lower surface ofthe device requires little movement. The small movement of the lowersurface of the device to make good electrical contact during testingdecreases the stress on the layers under the lower surface of thedevice. Accordingly, the likelihood of cracking the polyimide layers andthe conductive traces is reduced.

[0091] When probing an oxide layer on solder bumps, or solder balls onwafers that are to be used with “flip-chip” packaging technology, suchas the solder bumps on the printed circuit boards, the oxide layerdeveloped thereon is difficult to effectively penetrate. Referring toFIG. 24, when contacting a traditional contacting portion of a membraneprobe onto the solder bump, the oxide 285 tends to be pressed into thesolder bump 287 together with the contacting portion 289 resulting in apoor interconnection. When using conventional needle probes on solderbumps, the needles tend to skate on the solder bumps, bend under withinthe solder bumps, collect debris on the needles, flake the debris ontothe surface of the test device, and cleaning the needle probes is timeconsuming and tedious. Moreover, needle probes leave non-uniform probemarks on the solder bumps. When probing solder bumps used on flip-chips,the probe marks left in the upper portion of the solder bump tends totrap flux therein, which when heated tends to explode, which degrades,or otherwise destroys, the interconnection. Referring to FIGS. 25 and26, an improved device construction suitable for probing solder bumps isshown. The upper portion of the device includes a pair of steeplyinclined sides 291 and 293, such as 15 degrees off vertical, withpreferably polished sides. The inclined sides 291 and 293 preferablyform a sharp ridge 295 at the top thereof. The angle of the sides 291and 293 is selected with regard to the coefficient of friction betweenthe sides and the oxide on the solder bump, so that the oxide coatedsurface tends to primarily slide along the surfaces of the sides 291 and293, or otherwise shear away, and not be significantly carried on thesides as the device penetrates a solder bump. Referring to FIG. 27, thesubstantially sharp ridge also provides for a mark (detent) aftercontact that extends across the entire solder bump. Subsequent heatingof the solder bumps, together with flux, result in the flux exiting fromthe sides of the solder bump thereby avoiding the possibility ofexplosion. In addition, the resulting mark left on the solder bumps isuniform in nature which allows manufacturers of the solder bumps toaccount for-the resulting marks in their design. Also, less force isrequired to be applied to the device because it tends to slice throughthe solder bump rather than make pressing contact with the solder bump.The flatter surface 405 prevents slicing too deeply into the solder ball(bump).

[0092] Referring to FIG. 28, to provide a larger contact area fortesting solder bumps a waffle pattern may be used.

[0093] Referring to FIG. 29, an alternative device includes a pair ofprojections 311 and 313 that are preferably at the ends of an arch 315.The spacing between the projections 311 and 313 is preferably less thanthe diameter of the solder bump 317 to be tested. With such anarrangement the projections 311 and 313 will strike the sides of thesolder bump 317 thereby not leaving a mark on the upper portion of thesolder bump 317. With marks on the sides of the solder bump 317, thesubsequent flux used will be less likely to become trapped within themark and explode. In addition, if the alignment of the device is notcentered on the solder bump 317 then it is highly likely that one of theprojections 311 and 313 will still strike the solder bump 317.

[0094] Previous device construction techniques resulted in devices thatincluded contacting portions that were rather large and difficult toassure alignment of. Referring to FIG. 30, with the improvedconstruction technique the present inventors came to the realizationthat membrane probes may be used to make a “true” Kevlin connection to acontact pad on the test device. A pair of devices 351 and 353 arealigned with their contacting portions 355 and 357 adjacent one another.With this arrangement one of the devices may be the “force” while theother device is the “sense” part of the Kelvin testing arrangement. Bothcontacting portions 355 and 357 contact the same contact pad on the testdevice. A more detailed analysis of Kelvin connections is described inFink, D.G., ed., Electronics Engineers' Handbook, 1st ed., McGraw-HillBook Co., 1975, Sec. 17-61, pp. 17-25, 17-26, “The Kelvin DoubleBridge”, and U.S. patent application Ser. No. 08/864,287, both of whichare incorporated by reference herein.

[0095] It is to be noted that none to all of the aforementionedadvantages may be present in devices constructed accordingly to thepresent invention, depending on the technique used, desired use, andstructure achieved.

What is claimed is:
 1. A method of constructing a membrane probecomprising: (a) providing a substrate; (b) creating a depression withinsaid substrate; (c) locating conductive material within said depression;(d) connecting a conductive trace to said conductive material; and (e)removing said substrate from said conductive material.